502 DE. A. w. ROWE o:n the gekttjs micraster. [Aug. 1899^- 



sub-anal fascicle. The disposition of the oral series of avenues may- 

 be seen on PI. XXXIX, figs. 2 & 3. A glance at the figure of the 

 lateral avenue of Micraster cor-hovis (PL XXXIX, fig. 4) will show 

 that the paired pores are covered in by a strong arch. This feature 

 is best seen in this species on account of the smoothness of the test, 

 but it is invariably found in Micraster^ and is also seen in Berniaster, 

 Holaster, and Ecliinocorys. So far as Micraster is concerned, the 

 avenues become more obscure the higher one mounts in the zones, 

 on account of the progressive elaboration of the epistroma. 



The single ambulacrum of the superior surface ofi"ers a striking 

 similarity to the ambulacra of the inferior surface, for the ' granule ' 

 that separates the oblique paired pores appears in reality to be a 

 double arch, which is commonly fused by the taking-up of carbonate 

 of lime in fossilization, and so forms the so-called 'granule.' This 

 feature can be studied only in well-preserved examples, such as those 

 from Gravesend, and is clearly brought out in PL XXXYII, fig. 5. 

 It is, however, found in all species and varieties of Micraster, and its 

 absence is merely due to imperfect preservation. It will be seen that 

 the outer division of the arch is much thinner than the inner portion. 



In very well-preserved examples of M. cor-hovis one may detect 

 the distant ambulacral openings spreading down over the side of the 

 test from the end of the ambulacral groove. These pairs of pores 

 are separated by an arch, just as in the single ambulacrum. 



Between the two rows of pores in the case of the upper six or 

 seven pores of the paired ambulacra, a similar arch is thrown out. 

 This arch looks like a granule in most cases, but in rare instances 

 the double arch can be made out. After the sixth or seventh pore 

 it disappears as an arch, but it passes in the form of a trumpet- 

 shaped expansion over the outer side of the inner row of pores. 

 This feature is seen in PL XXXYI, figs. 5 & 6. 



Single ambulacrum. — Though not nearly so useful a zonal 

 guide as the paired ambulacra, the single ambulacra show conclu- 

 sively that they also share in the progressive evolution of the test. 



In PL XXXVII, fig. 1, is seen the feebly granular and almost 

 smooth interporiferous area of M. cor-hovis, while fig. 2 gives a slight 

 increase of granulation in the case of M. Leshei, neither of them 

 showing the least trace of the sutures of the plates. Fig. 3, from 

 an example of M. prcecursor in the H. planus-ione, again leads up' 

 to a definite granulation and suturing of the central area, which 

 is intensified in fig. 4, representing an example of M. prcecursor 

 from the M. cor-testuclinarium -zorie ; and the climax is reached when 

 we examine fig. 5, taken from a beautifully-preserved specimen 

 of M. cor-anguinum from the zone of the same name. In this 

 last we notice the double arches between the pores, the increased 

 granulation of the interporous ridge and of the interporiferous 

 plates, with a heaping-up of the edges of the latter plate in the 

 middle line. The paired pores are quite sunk by this elaboration' 

 of the epistroma in the lower end of the ambulacrum. 



No name has been given to the diff'erent varieties of the single^ 



